1. Field
Some embodiments described herein relate generally to radiation treatment, and more particularly, to methods, apparatus and computer readable mediums for use in accounting, at least in part, for changes in a position of a tumor or other target volume within a patient.
2. Description
According to conventional radiation therapy, a beam of radiation is directed toward a tumor located within a patient. The radiation beam delivers a predetermined dose of therapeutic radiation to the tumor according to an treatment plan. The delivered radiation kills cells of the tumor by causing ionizations within the cells.
Recent advances in fractionated external beam radiation therapy, such as three-dimensional conformal and intensity-modulated radiation therapy (IMRT), have increased the ability to deliver radiation doses that conform tightly to a target volume. This tight conformance results in steep dose gradients inside the volume. For example, IMRT can create a dose gradient of 10% mm−1 inside a target volume.
A treatment plan is designed assuming that a target volume will be in a particular position relative to a beam source during treatment. If the target volume is not positioned exactly as assumed by the treatment plan, the steep gradient may occur within sensitive healthy tissue surrounding the volume causing destruction of healthy tissue while sparing some malignant tissue. Thus, it is increasingly important to precisely position the target volume with respect to the beam source.
It is not unusual for the target volume to change position within the patient (e.g., to translate along one or more axes and/or rotate about one or more axes) after a treatment plan is designed but prior to performing the treatment.
In order to know current location of the target volume with respect to the external beams, three-dimensional imaging of the patient is often provided immediately prior to treatment delivery (i.e., when the patient is on the treatment table). Systems attempting to provide such imaging include: (1) a “CT on rails” system, requiring an additional diagnostic computed tomography machine in the treatment room; (2) a kilovoltage cone beam CT (kVCBCT) system, consisting of an additional kilovoltage X-ray source and detector attached to a treatment gantry; (3) a megavoltage cone beam CT (MVCBCT) system using the pre-existing treatment machine and an EPID for imaging; (4) a MVCT system, using the pre-existing treatment machine with an attached arc of detectors; (5) a tomotherapy system, replacing the traditional treatment machine with a CT ring and a MV beam source; and (6) a pre-treatment magnetic resonance imaging (MRI) of the patient.
From pre-treatment imaging, a shift of the target volume with respect to the external beams can be found and the patient position is adjusted in order to position the tumor targets in the intended planned position with respect to the external beams. Typically, the shift of such target volumes can be modeled as rigid body rotation around along three orthogonal axis and rigid body translation along three orthogonal axis. Adjustments of the patient position typically involve movement of a radiotherapy couch.
For example, if the treatment system uses a robotic couch having six degrees of freedom (e.g., translation along three axes and rotation about three axes), the patient may be placed on the robotic couch and the couch may be actuated so as to move the patient to a position at which the tumor has a position, relative to the treatment system, that is the same as that used in defining the radiation treatment plan.
If the treatment system uses a table having only four degrees of freedom (e.g., translation along three axes and rotation about one axis), the positions of the table, the gantry and the collimator may be each adjusted for each planned beam, such that the tumor, the gantry and the collimator have the same relative positioning as defined by the treatment plan. (See Yue et al., A method to implement full six-degree target shift corrections for rigid body in image-guided radiotherapy. Medical Physics, 33(1):21-31, January 2006.)
Alternately, without moving a treatment couch, the collimator may be rotated and leaves and jaws of the collimator for a beam may be repositioned to match a current position and shape of the target volume with respect to the beam and the dose is then recomputed. (See Ludlum et al., An algorithm for shifting MLC shapes to adjust for daily prostate movement during concurrent treatment with pelvic lymph nodes, Med Phys. 2007 December; 34(12):4750-6. See also Erik-Jan Rijkhorst et al. Strategy for online correction of rotational organ motion for intensity-modulated radiotherapy of prostate cancer. International Journal of Radiation Oncology*Biology*Physics, 69:1608-1617, 2007).